Search

A series of regioisomeric push–pull amino-nitro [6]helicenes and a related [7]helicene derivative were prepared and their racemates resolved into enantiomers. Compared to the parent helicenes, they exhibit red-shifted UV-Vis spectra, pronounced dipole moments, altered chiroptical properties such as remarkable optical rotatory power, and can form nanocrystalline Langmuir–Blodgett films. [ABSTRACT FROM AUTHOR]

The synthesis of a tetrathiafulvalene (TTF) derivative, S-[4-({4-[(2,2′-bi-1,3-dithiol-4-ylmethoxy)methyl] phenyl}ethynyl)phenyl] ethanethioate, suitable for the modification of gold nanoparticles (AuNPs), is described in this article. The TTF ligand was self-assembled on the AuNP surface through ligand exchange, starting from dodecanethiol-stabilized AuNPs. The resulting modified AuNPs were characterized by TEM, UV-Vis spectroscopy, and electrochemistry. The most suitable electrochemical method was the phase-sensitive AC voltammetry at very low frequencies of the sine-wave perturbation. The results indicate a diminishing electronic communication between the two equivalent redox centers of TTF and also intermolecular donor–acceptor interactions manifested by an additional oxidation wave upon attachment of the ligand to AuNPs. [ABSTRACT FROM AUTHOR]

Racemic and highly enantioenriched 3‐methoxycarbonyl, 3‐carboxy, and 3‐hydroxymethyl derivatives of dibenzo[6]helicene were prepared. The Langmuir layers of these helicenes were formed at the air–water interface and transferred onto solid substrates to afford Langmuir–Blodgett films, which were then studied by ambient atomic force microscopy and (chir)optical spectroscopy. Significant differences were found in the behaviour of the Langmuir layers as well as in the morphology, UV/Vis, electronic circular dichroism (ECD), and fluorescence spectra of the Langmuir–Blodgett thin films depending on the molecular chirality and nature of the polar group. The experimental results were supported by molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

Chimerical pyrene-based dibenzo[7]helicene rac- 1 and 2 H-pyran[7]helicene ( M, R, R)-(−)- 2, in which two pyrene subunits are fused to the [7]helicene/[7]heterohelicene scaffold, were synthesised by means of Ni0- or CoI-mediated [2+2+2] cycloisomerisation of dipyrenyl-acetylene-derived triynes. Pyrene-based dibenzo[7]helicene 1 was obtained in enantioenriched form by enantioselective cycloisomerisation under Ni0/QUINAP catalysis (57 % ee) or in enantiopure form by racemate resolution by liquid chromatography on a chiral column. 1,3-Allylic-type strain-controlled diastereoselective cycloisomerisation was employed in the synthesis of enantiopure ( M, R, R)-(−)- 2. Physicochemical properties of 1 and 2 encompassing the helicity assignment, stability to racemisation, X-ray crystal structure, UV/Vis, experimental/calculated electronic circular dichroism and fluorescence spectra were studied. Accordingly, comparison of the X-ray crystal structure of ( M, R, R)-(−)- 2 with calculated structures (DFT: B3LYP/cc-pVDZ, B97D/cc-pVDZ) indicated that its helical backbone is slightly over-flattened owing to intramolecular dispersion forces between tert-butylated pyrene subunits. Both 1 and 2 are fluorescent (with quantum yields in dichloromethane of ΦF=0.10 and 0.17, respectively) and are suggested to form intramolecular excimer states upon excitation, which are remarkably stabilised and exhibit large Stokes shifts (296 and 203 nm, respectively). [ABSTRACT FROM AUTHOR]

The article discusses the rapid access and functionalized derivatives of dibenzohelicenes. It states that the dibenzohelicenes' construction relies on the pivotal Sonogashira and Suzuki-Miyaura couplings for the assembly of key aromatic triynes with a subsequent alkyne cycloisomerization for the formation of helical backbones. It says that the dibenzohelicenes have an advantage over helicenes due to the simplicity of its non-photochemical preparation.

The article discusses the rapid access and functionalized derivatives of dibenzohelicenes. It states that the dibenzohelicenes' construction relies on the pivotal Sonogashira and Suzuki-Miyaura couplings for the assembly of key aromatic triynes with a subsequent alkyne cycloisomerization for the formation of helical backbones. It says that the dibenzohelicenes have an advantage over helicenes due to the simplicity of its non-photochemical preparation.

Abstract: A large selection of triptycenes functionalized with tetrathiafulvalene (TTF) units as well as triptycenes containing extended TTFs as a part of the triptycene core have been synthesized utilizing new triptycene di- and tetraaldehydes as well as bis-, tetrakis- and hexakis(bromomethyl) derivatives. The largest scaffold contains a total of 12 TTFs around the central triptycene core. From spectroelectrochemical and chemical oxidation studies, we have elucidated the extent to which an increasing number of electrostatic interactions among oxidized TTF units exert an influence on the absorption characteristics. [Copyright &y& Elsevier]

Molecular platforms are essential components of various surface‐mounted molecular devices. Here, we document the synthesis of two universal triptycene‐based tripodal pedestals featuring terminal alkynes in the axial position. We showcase their versatility by incorporating them into the structures of diverse functional molecules such as unidirectional light‐driven molecular motors, photoswitches, and Brownian molecular rotors using standard cross‐coupling reactions. We also present their fundamental physical properties, including acidity constants, data from differential scanning calorimetry, and crystallographic analysis of two parent and five derived structures. Finally, and importantly, we demonstrate that the photochemical properties of selected photoswitch representatives remain uncompromised when fused with tripods. [ABSTRACT FROM AUTHOR]

Cholesterol plays an important biological role in the body, and its disruption in homeostasis and synthesis has been implicated in several diseases. Mapping the locations of cholesterol is crucial for gaining a better understanding of these conditions. Silver deposition has proven to be an effective method for analyzing cholesterol using mass spectrometry imaging (MSI). We optimized and evaluated thermal evaporation as an alternative deposition technique to sputtering for silver deposition in MSI of cholesterol. A silver layer with a thickness of 6 nm provided an optimal combination of cholesterol signal intensity and mass resolution. The deposition of an ultrathin nanofilm of silver enabled high-resolution MSI with a pixel size of 10 µm. We used this optimized method to visualize the distribution of cholesterol in the senile plaques in the brains of APP/PS1 mice, a model that resembles Alzheimer's disease pathology. We found that cholesterol was evenly distributed across the frontal cortex tissue, with no evidence of plaque-like accumulation. Additionally, we investigated the presence and distribution of cholesterol in myocardial sections of a human heart affected by wild-type ATTR amyloidosis. We identified the presence of cholesterol in areas with amyloid deposition, but complete colocalization was not observed. [ABSTRACT FROM AUTHOR]

Invited for the cover of this issue is Bernd Schmidt and co‐workers at University of Potsdam and Irena G. Stará, Ivo Starý and co‐workers at IOCB Prague. The image depicts two assemblies of an amphiphilic helicene molecule that float on the water surface and differ in chirality: The racemic "raft" is more stable than the enantiopure one. Read the full text of the article at 10.1002/chem.201901695. [ABSTRACT FROM AUTHOR]

Both experiments and molecular dynamics simulations point to the fact that racemic assemblies are more stable than enantiopure ones. B Helicene rafts on water b : Amphiphilic derivatives of dibenzo[6]helicene form stable Langmuir layers at the air-water interface. Both experiments and molecular dynamics simulations point to the fact that racemic assemblies are more stable than enantiopure ones. [Extracted from the article]

2,9‐ and 2,10‐diphenylpentacene were synthesized by direct C−H borylation of ketal‐protected pentacene, followed by halodeboronation, resolution of the dihalo isomers, Suzuki arylation, cleavage of the ketals and decarbonylation in the solid state. They were studied as main active components in organic field effect transistors (OFETs). Diphenyl substitution of pentacene affects the unit cell dimensions only slightly, preserving a face to edge molecular packing in the first layers of thin films evaporated on SiO2 substrates. Both isomers self‐assemble into nanoribbons during the thin film growth upon vapor deposition. The similarity between the surface induced phases of the 2,9‐isomer and unsubstituted pentacene leads to similar 4‐probe hole mobilities, i. e. 0.13 cm2 V−1 s−1 for the former. Whereas 2,9‐disubstitution thus does essentially preserve the thin film characteristics of unsubstituted pentacene, 2,10‐disubstitution is detrimental to the molecular ordering in the thin films and therefore to the field effect mobility which is only 0.07 cm2 V−1 s−1. The known strong enhancement of field effect mobility observed upon diphenyl substitution of anthracene can thus not be emulated analogously with pentacene. [ABSTRACT FROM AUTHOR]

A series of oxahelicenes composed of ortho/ meta-annulated benzene/pyridine and 2 H-pyran rings were synthesized on the basis of the cobalt(I)-mediated (or rhodium(I)- or nickel(0)-mediated) double, triple, or quadruple [2+2+2] cycloisomerization of branched aromatic hexa-, nona-, or dodecaynes, thus allowing the construction of 6, 9, or 12 rings in a single operation. The use of a flow reactor was found to be beneficial for the multicyclization reactions. The stereogenic centers present in some of the oligoynes steered the helical folding in such a way that the final oxa[9]-, [13]-, [17]- and [19]helicenes were obtained in both enantiomerically and diastereomerically pure form. Specifically, the oxa[19]helicenes beat the current record in the length of a helicene backbone. Single-molecule conductivity was studied by the mechanically controllable break-junction method with a pyridooxa[9]helicene. [ABSTRACT FROM AUTHOR]

A series of oxahelicenes composed of ortho/ meta-annulated benzene/pyridine and 2 H-pyran rings were synthesized on the basis of the cobalt(I)-mediated (or rhodium(I)- or nickel(0)-mediated) double, triple, or quadruple [2+2+2] cycloisomerization of branched aromatic hexa-, nona-, or dodecaynes, thus allowing the construction of 6, 9, or 12 rings in a single operation. The use of a flow reactor was found to be beneficial for the multicyclization reactions. The stereogenic centers present in some of the oligoynes steered the helical folding in such a way that the final oxa[9]-, [13]-, [17]- and [19]helicenes were obtained in both enantiomerically and diastereomerically pure form. Specifically, the oxa[19]helicenes beat the current record in the length of a helicene backbone. Single-molecule conductivity was studied by the mechanically controllable break-junction method with a pyridooxa[9]helicene. [ABSTRACT FROM AUTHOR]

This article discusses the synthesis of triptycene-based tripodal molecular platforms, which are important for surface-mounted molecular devices. These molecular tripods have been shown to be versatile by being integrated into various functional molecules, such as molecular motors, photoswitches, and molecular rotors. The researchers achieved this by using standard cross-coupling reactions. For more information, readers can refer to the research article by J. Kaleta and co-workers. [Extracted from the article]

The role of the helicity of small molecules in enantioselective catalysis, molecular recognition, self-assembly, material science, biology, and nanoscience is much less understood than that of point-, axial-, or planar-chiral molecules. To uncover the envisaged potential of helically chiral polyaromatics represented by iconic helicenes, their availability in an optically pure form through asymmetric synthesis is urgently needed. We provide a solution to this problem present since the birth of helicene chemistry in 1956 by developing a general synthetic methodology for the preparation of uniformly enantiopure fully aromatic [5]-, [6]-, and [7]helicenes and their functionalized derivatives. [2 + 2 + 2] Cycloisomerization of chiral triynes combined with asymmetric transformation of the first kind (ultimately controlled by the 1,3-allylic-type strain) is central to this endeavor. The point-to-helical chirality transfer utilizing a traceless chiral auxiliary features a remarkable resistance to diverse structural perturbations. [ABSTRACT FROM AUTHOR]

A series of inherently chiral (di)thiahelicenes/dithiophenohelicenes containing 5, 6, 9 or 11 (hetero)cycles in their helical backbone with thiophene subunit(s) embedded in it or annulated to its periphery were uniformly synthesized by intramolecular alkyne [2+2+2] cycloisomerization. Their structural, chemical and physicochemical properties such as crystal packing, oxidation, barrier to racemization, spontaneous thin film transformation or electrochemical behavior were studied and compared with the respective properties of the parent all‐carbon helicenes and dibenzohelicenes. [ABSTRACT FROM AUTHOR]

The tetraene 7,7‐dimethoxy‐2,3,5,6‐tetramethylenebicyclo[2.2.1]heptane is a key compound for the preparation of a large variety of acenes protected by a carbonyl bridge. We report herein a medium scale preparation in seven steps of this valuable starting material. Diels–Alder addition between 6,6‐dimethtyl fulvene and maleic anhydride, followed by carboxylation, ozonolysis of the double bond, reduction of the four ester groups, then chlorination of the alcohol groups and dehydrochlorination give the target compound in 17 % overall yield. [ABSTRACT FROM AUTHOR]

Abstract: Helical chirality is a novel enantioselectivity‐inducing property in transition‐metal‐catalyzed transformations. The principle is illustrated herein for the example of asymmetric olefin metathesis. This work reports the synthesis of the first helically chiral Ru‐NHC alkylidene complex from an aminohelicene‐derived imidazolium salt, which was ligated to the first generation Hoveyda–Grubbs catalyst. Kinetic data were acquired for benchmark test reactions and compared to an achiral catalyst. The helically chiral Ru‐catalyst was evaluated in asymmetric ring‐closing metathesis (RCM) and ring‐opening metathesis–cross‐metathesis (ROM/CM) reactions, which proceeded with promising levels of enantioselectivity. Extensive NMR‐spectroscopic investigations and a DFT geometry optimization were performed. These results led to a topographic steric map and calculation of percent‐buried‐volume values for each quadrant around the metal center. [ABSTRACT FROM AUTHOR]

Termite soldiers often combine mechanical adaptations with defensive chemicals secreted from the frontal gland. Amongst the most remarkable strategies for mechanical defence, symmetrical and asymmetrical snapping mandibles evolved in several lineages of the diversified subfamily Termitinae (Termitidae). The contribution of the frontal chemical weapon to defence in snapping soldiers has long been doubted and the subfamily Termitinae overlooked with respect to soldier-produced chemicals. We recently reported an active frontal gland secreting unique defensive chemicals in the symmetrically snapping soldiers of Cavitermes tuberosus. The aim of the present study was a larger-scale comparison of chemical defence in symmetrically snapping soldiers. We studied the anatomy of the frontal gland and the chemistry of its secretion in five additional Neotropical species and mapped our observations on a de novo constructed molecular phylogeny of the target group. We show that the soldiers of all studied species possess a functional frontal gland, housed in part in the frontal projections on their heads. Phylogenetic reconstruction groups the studied taxa into two well-defined clades, supported by fundamental differences in defensive chemicals, either arising exclusively from the lipogenic pathway or containing also the products of the isoprenoid pathway. Our results also identify a new genus of symmetrical snappers, related to the genus Cavitermes, incorrectly classified in several previous studies. [ABSTRACT FROM AUTHOR]

A straightforward approach to enantiopure 2H-pyran-modified amino[5]helicenes and amino[6]helicenes was developed. They were converted to 1,3-disubstituted imidazolium salts and used as NHC ligand precursors in the Ni0-catalysed enantioselective [2+2+2] cycloisomerisation of aromatic triynes to obtain the model helicene derivatives in up to 86% ee. [ABSTRACT FROM AUTHOR]

We report on a new analogue of locked cyclohexane nucleic acids that contains an adenine nucleobase at an alternative position and which resembles homo-DNA. The crucial locked carbocyclic nucleoside monomer was prepared with high enantioselectivity and was subsequently incorporated into a pair of modified oligonucleotides that were subjected to hybridization studies. [ABSTRACT FROM AUTHOR]

Taurine (2-aminoethanesulfonic acid) is a representative substructure in biologically active compounds, but can raise difficulties in direct coupling reactions. In this study, the synthesis of taurine-derived sulfonic acids and analogous aromatic sulfonic acids was accomplished by a performic acid promoted oxidation of corresponding symmetrical disulfide precursors. In some of the products, the taurine or sulfanilic acid nitrogen atom is incorporated in a heterocyclic scaffold or part of a peptide bond in dipeptide mimetics. [ABSTRACT FROM AUTHOR]

A cobalt-mediated [2+2+2] cycloisomerisation of ynedinitriles to helical pyridazines in good to high yields was developed. The construction of the pyridazine nucleus from one alkyne and two nitrile units is proposed to follow either a conventional organometallic mechanism or to be triggered by a single-electron transfer from a CoII species. Various [5]-, [6]- and [7]helicene pyridazines were prepared. [ABSTRACT FROM AUTHOR]